Typically, during materials processing, plasma is employed to facilitate the partial or complete addition and removal of thin films. For example, in semiconductor processing, a (dry) plasma etch process is used to remove or etch material along fine trenches or within vias or contacts patterned on a silicon substrate. Alternatively, a vapor deposition process is used to deposit material along fine lines or within vias or contacts on a silicon substrate. In the latter, vapor deposition processes include thermal chemical vapor deposition (CVD) and plasma enhanced chemical vapor deposition (PECVD).
In PECVD, plasma is used to alter or enhance the film deposition mechanism. For instance, plasma excitation generally allows film-forming reactions to proceed at temperatures that are significantly lower than those typically required to produce a similar film by thermally excited CVD. In addition, plasma excitation may activate film-forming chemical reactions that are not energetically or kinetically favored in thermal CVD. The chemical and physical properties of PECVD films may thus be varied over a relatively wide range by adjusting process parameters.
More recently, atomic layer deposition (ALD), a form of CVD or more generally film deposition, has emerged as a candidate for ultra-thin gate film formation in front end-of-line (FEOL) operations, as well as ultra-thin barrier layer and seed layer formation for metallization in back end-of-line (BEOL) operations. In ALD, two or more process gases are introduced alternately and sequentially to form a material film one monolayer (or less) at a time. Such an ALD process has proven to provide improved uniformity and control in layer thickness, as well as conformality to features on which the layer is deposited.
The introduction of copper (Cu) metal into multilayer metallization schemes for manufacturing integrated circuits has necessitated the use of diffusion barriers/liners to promote adhesion and growth of the Cu layers and to prevent diffusion of Cu into the dielectric materials. Barriers/liners that are deposited onto dielectric materials can include refractive materials, such as tungsten (W), molybdenum (Mo), and tantalum (Ta), that are non-reactive and immiscible in Cu, and can offer low electrical resistivity. For example, Cu integration schemes for technology nodes less than or equal to 130 nm can utilize a low dielectric constant (low-k) inter-level dielectric, followed by a physical vapor deposition (PVD) of a Ta film or a TaN/Ta film, followed by a PVD Cu seed layer, and an electro-chemical deposition (ECD) Cu fill. Generally, Ta-containing films are chosen for their adhesion properties (i.e., their ability to adhere on low-k films) and their barrier properties (i.e., their ability to prevent Cu diffusion into the low-k film).
The deposition of a Ta-containing film using PVD can be problematic for structures with high aspect ratios, the ratio of height-to-width of a trench for example, where the PVD deposition of the Ta-containing film is non-uniform along the side walls and bottom of the trench. In one example, a thick film layer may be formed on the bottom of the trench while the film layer on the sidewalls of the trench may be very thin or even non-existent.